Turquoise

Turquoise is a hydrated copper aluminium phosphate. The copper supplies the blue, and iron substituting for aluminium pulls it toward green, which is why turquoise ranges continuously from sky blue to yellowish green and why the greener material tends to come from iron-rich deposits.

It is a comparatively soft and porous stone, around 5 to 6 on the Mohs scale, and this is the central practical fact about it. Turquoise absorbs oils, cosmetics and skin secretions, and it changes colour with wear: drifting green, sometimes over years, sometimes rapidly. The old belief that turquoise warns its owner of illness or danger by changing colour has an obvious physical basis: the stone genuinely does change, in response to the wearer's body chemistry and to what it has been exposed to. A belief that looks superstitious turns out to describe a real and observable behaviour, while the causal story attached to it is wrong.

Because of the porosity, most commercial turquoise is stabilised, impregnated with resin to harden it and fix the colour. Untreated high-grade material is a small fraction of the market. Reconstituted turquoise, dyed howlite and dyed magnesite are all sold as turquoise, and misrepresentation is common.

The matrix, the dark veining, is the surrounding host rock. Spiderweb turquoise, with fine even veining, is valued in some markets and considered a defect in others; Persian taste historically favoured pure unveined blue, and Southwestern American taste often prizes the matrix.

The name is French, pierre turquoise, Turkish stone, not because it came from Turkey but because it reached Europe through Turkish trade from Persian sources.

Turquoise has three substantially independent traditions, and they should not be merged into a single body of lore.

Egypt mined turquoise in the Sinai from the Old Kingdom onward, at Serabit el-Khadim and Wadi Maghareh, where the mining expeditions left inscriptions and a temple to Hathor, called the Lady of Turquoise. This is among the oldest documented organised gem mining anywhere, and the turquoise is found in Egyptian jewellery from the earliest dynasties.

Persia produced the finest material from the mines near Nishapur, worked for well over a thousand years, and the intense unveined blue of that source set the standard the word turquoise implies. Persian turquoise was traded across Asia and into Europe, and the horseman's practice of fixing turquoise to bridles as a guard against falls is attested across the Persian and Turkic world.

The American Southwest is a wholly separate tradition. Turquoise was mined and worked at Cerrillos in New Mexico and elsewhere long before European contact, traded into Mesoamerica, and is central to Puebloan, Navajo and Apache ritual and ornament, and to Aztec mosaic work. The silver-and-turquoise jewellery now universally identified with the region is a later development: Navajo silversmithing begins in the mid-nineteenth century, learned from Mexican smiths, and the combination with turquoise develops from there, expanding enormously with the tourist trade from the 1880s.

Turquoise is the December birthstone. On the 1912 list December was lapis lazuli; turquoise and zircon came in with the 1952 revision, tanzanite in 2002.